In x-ray imaging, metrology and spectroscopy systems, there is often a need to achieve emission of x-ray beams with relatively higher x-ray energy, that is, with shorter x-ray wavelength. Such beams can provide improved resolution-ray penetration, and hence improved contrast and resolution, especially when used in imaging apparatuses, and particularly in microfocus imaging apparatuses.
In x-ray emission apparatuses, x-ray emission is achieved by bringing a beam of accelerated electrons into interaction with a target of an x-ray generating material, usually a metal with a relatively high atomic number (Z) such as tungsten. The electrons are accelerated by emission from a source of relatively more negative electrical potential than the target, such that the electrons emitted from the source accelerate away from the source toward the target. Thermionic emission, for example, may be used to generate appropriate electrons for acceleration.
Electron beam generation and x-ray emission is usually performed under high vacuum conditions, because the presence of air in an electron beam apparatus can cause absorption of the electron beam and can prevent the maintenance of the high potential differences required to produce high-energy electrons, and thereby x-rays. However, even in an ultra-high vacuum system, there is a difficulty in achieving increasingly greater accelerating potentials, because increasing the potential of the source relative to the walls of the vacuum chamber in which it is enclosed increases the risk of vacuum breakdown and dissipation of the high potential difference, leading to failure. This can be mitigated to some degree by increasing the size of the vacuum chamber, but this renders the apparatus bulky, expensive and difficult to manufacture.
Accordingly, it has been proposed in a modified form of x-ray system to have a high negative potential difference between the electron source and the walls of the vacuum chamber and a high positive potential difference between the walls of the vacuum chamber and the x-ray target. In such a design, sometimes called a bipolar system, the electron beam is not only accelerated away from the electron source, but is accelerated toward the target. The total accelerating potential is the difference in potential between the source and the target, but the apparatus can be smaller as compared with a conventional apparatus because the potential difference between each of i) the source and the chamber and ii) the chamber and the target is much less than the total accelerating potential. Accordingly, the risk of vacuum breakdown is mitigated. Further, a magnetic focussing lens that is conventionally held at ground potential may be interposed in the beam tunnel between the negative cathode electrode and the positive target.
However, in realising such configurations, there has been a problem in stability of the positive part of the apparatus, namely that portion of the apparatus which contains the high-voltage target.
A candidate configuration for such a target assembly is shown in cross-section in FIG. 1. In FIG. 1, target assembly 90 has a vacuum chamber 91 which defines an enclosure for the target apparatus. Vacuum chamber 91 is adapted to maintain a sufficiently high vacuum, typically 10−5 mbar or better. Such vacuums may be achieved by ensuring that the enclosure is suitably vacuum-sealed, and then by applying a suitable vacuum pump, such as a turbo pump, to a pump port (not shown). High vacuum is necessary to support the electron beam.
The vacuum chamber 91 is held at ground potential, by a connection to ground (not shown).
At least one wall 92 is conductive, and advantageously the entire enclosure is conductive to avoid static accumulation. A suitable conductive material for forming the at least one conductive wall 92, and also the whole vacuum chamber 91, is aluminium.
A slightly tapered, rod-like insulating element 93 projects through conductive wall 92 of vacuum chamber 91. Insulating element 93 may be formed, for example of an insulating resin such as epoxy resin or polyetherimide (PEI) resin. Insulating element 93 contains a high voltage conductor 94 arranged coaxially with the insulating elements, which may be connected to a high voltage supply positioned outside chamber 91.
In the configuration shown in FIG. 1, insulating element 93 and conductor 94 each have a two-part construction, to enable easy coupling and decoupling of the chamber from the high voltage source. Insulating element 93 may, for example, be formed by a combination of a first tapered rod, having an internal tapered cavity formed within the first tapered rod, and a second tapered rod having external taper to match the internal taper of the first tapered rod so as to be accommodated within the first tapered rod. The conductor 94 may, for example, then be provided with a first part in the second tapered rod, and a second part within the first tapered rod. The first and second parts of the conductor may mate via a conductive coupler when the second tapered rod is accommodated in the cavity of the first tapered rod. However, such a configuration is not essential, and insulating element 93 and conductor 94 can each be of unitary construction.
Insulating element 93 supports, at an end portion 93a which is furthest from conductive wall 92, target housing 95. Target housing 95 is electrically connected to high voltage conductor 94. The high voltage carried on conductor 94 is exposed to the vacuum contained within chamber 91 at this point. Housing 95 supports x-ray generating target 96 and elevates x-ray generating target 96 to the high potential of conductor 94 by providing an electrical connection between conductor 94 and target 96.
In this configuration, housing 95 is made of a radiodense material, for example an 80% tungsten/20% copper alloy. Housing 95 has a cone-shaped opening to allow the generated x-rays, which have been generated by x-ray generating target 96, to emerge. This approach is able to limit the x-rays to a cone-shaped beam that is just large enough to illuminate a detector with which the apparatus is intended to operate at its intended position and orientation. Such an approach may reduce unwanted x-ray scatter, which may improve contrast. Such an approach may also reduce the thickness of any shielding need for parts of the apparatus that are not arranged along the direction of x-ray beam X.
The cone-shaped aperture may be closed by a thin transparent window, formed of, for example, a thin sheet of radiolucent material such as aluminium or beryllium to avoid gas, which has been generated by x-ray generating target 96 under irradiation by electron beam E, being ejected into the space between target housing 95 and an opposing wall of chamber 91, in which space a high electric field may be present. Such an approach may also therefore improve stability against gas-induced vacuum breakdown.
In this configuration, the target housing 95 is also provided with an entrance tunnel through which the electron beam E is able to reach the x-ray generating target 96. The entrance tunnel may have a deliberately reduced diameter. Such a configuration may provide a throttle to impede the gas which may be ejected from x-ray generating target 96 as described above.
Chamber 91 has an x-ray emission window 97 arranged adjacent to x-ray generating target 96 so that x-rays X generated from the target can exit the chamber while preserving the high vacuum in the chamber. Such a window may be made, for example of a thin sheet of a material which is radiolucent (or transparent to x-rays) such as aluminium or beryllium. Target 96 is made of a high-atomic number (high-Z) material such as tungsten, which is able to generate x-rays when irradiated with a suitably high-energy electron beam.
Chamber 91 also has an electron beam acceptance aperture 98 through which an electron beam E may be introduced so as to impinge on x-ray generation target 96. Electron beam acceptance aperture 98 may have a mounting arrangement, not shown, adapted to couple target assembly 90 to an electron-beam gun so as to form a unitary vacuum chamber in a so-called two-arm arrangement. Such a mounting arrangement may include, for example, high vacuum seals arranged between an exit port of the electron-beam generator and beam introduction aperture 98 of target assembly 90.
In operation, target assembly 90 of FIG. 1 accepts an electron beam through aperture 98, which impinges on target 96, thereby generating x-rays X which are emitted through window 97. Target 96 is maintained at an elevated voltage via the electrical connection, through target housing 95, with conductor 94, which is supported within vacuum chamber 91 by insulating element 93 which extends through conductive wall of the vacuum chamber 91. By such an arrangement, the incident electron beam through aperture 98 can be further accelerated by the high positive potential of target 96 derived from conductor 94. Higher-energy X-rays may thereby be produced.
However, the configuration shown in FIG. 1 may exhibit a disadvantage in that, when the conductor 94 carries a high positive potential, a high potential gradient exists between conductor 94 and the surrounding chamber 91, especially conductive wall 92. Although insulating element 93 prevents the vacuum enclosed within vacuum chamber 91 from contacting conductor 94, and hence isolates conductor 94 from the vacuum, electrons are emitted from the most negative surface in the chamber, which electrons can multiply or avalanche as they interact with the surface of the insulating element that separates the most positive electrode, namely conductor 94, from the rest of the chamber. These processes can lead to an ionised path being created that allows a high voltage breakdown, with a convergent rapid discharge of the energy stored within the high-voltage-generating elements of the target assembly. In the configuration of FIG. 1, the conductive wall 92 of the housing, at least, acts as a strongly negative electrode creating a very large area that can provide a copious source of electrons.
Especially, at the interface T between i) the insulating element 93, ii) the metal wall 92 of the vacuum chamber, and iii) the vacuum, the potential barrier is lower and electrons easily escape from the metal into the vacuum. These electrons are accelerated towards the insulating element surface where they accumulate, causing the insulating element surface to become locally negatively charged, but also causing the release of multiple secondary electrons, especially if the incident electrons have energy significantly above 100 eV. These secondary electrons are also accelerated and cause further charging of the insulating element, as they “hop” progressively along the length of insulating element 93 towards target housing 95. This process leads to surface degassing of insulating element 93. The local gas cloud so produced may eventually become ionised by the avalanche electrons, creating a gas plasma channel through which the stored electrical energy and the high voltage system may suddenly and violently be discharged.
Such a discharge inhibits the maintenance of a stable high voltage source, and may be highly damaging to the apparatus.
Accordingly, there is a requirement for an improved target assembly which is able to inhibit such processes and which is able to maintain a high, stable, positive potential between the target and the enclosing vacuum chamber.